In the recent years, polyimide resin materials are used as protective films on semiconductor elements such as transistors, IC'S and LSI's as well as interlayer insulating films between multi-layer wirings. The polyimide resin materials utilized for such purposes are subject to fine working steps of forming through holes or the like as always required in the manufacture of semiconductor elements. For the purpose of completing the fine working process more effective, attempts were made to develop polyimide resin materials which could also be used as photosensitive heat-resistant material.
There were proposed several materials including
(1) a composition comprising a polyamic acid and a bichromate (JP-B No. 17374/1974), PA1 (2) a composition comprising a compound obtained by introducing a photosensitive group into the carboxyl group of a polyamic acid through ester linkage (JP-B 30207/1980), PA1 (3) a composition comprising a compound obtained by reacting a polyamic acid with an epoxy-containing compound having a photosensitive group (JP-A 45746/1980), and PA1 (4) a composition comprising a polyamic acid and an amine compound having a photosensitive group (JP-B 145794/1979). It is to be noted that Japanese Patent Publication is abbreviated as JP-B and Japanese Patent Application Kokai as JP-A.
Composition (1) has a very short pot life and forms Polyimide films having chromium ion left therein. Composition (2) forms polyimide films having chloride ion left therein because a dehydrochlorination reaction is used to introduce a photosensitive group. Therefore films of both compositions (1) and (2) suffered from a purity problem. In composition (3), a photosensitive group can be introduced into the base compound through ester linkage without contaminating it with an ionic impurity, but at a very low rate of introduction which leads to low sensitivity in practical applications. Composition (4) is advantageous in that no ionic impurities are introduced and a sufficient amount of photosensitive group can be introduced, but has some disadvantages. In a prebaking step of volatilizing the solvent from a photosensitive Polymeric solution before exposure, the photosensitive component can be partially volatilized off. A problem also arises in forming thick films. As the solvent volatilizes off in the prebaking step, the photosensitive component becomes a poor solvent for the polyamic acid, causing the resulting films to be whitened leading to difficulty in subsequent steps.